In recent years, as portable electronic devices such as mobile telephones, notebook personal computers and the like have spread, there is a large need for development of compact and lightweight secondary batteries having high energy density. There is also a large need for development of high-output secondary batteries as the batteries of electric automobiles, such as hybrid automobiles. As a kind of non-aqueous electrolyte secondary battery that satisfies such a need is a lithium-ion secondary battery. A lithium-ion secondary battery comprises an anode, a cathode and an electrolyte; and as the active material of the anode and cathode, a material from which lithium can be removed or inserted is used.
Currently, research and development of this kind of lithium-ion secondary battery is actively being carried out, however, of this research and development, in order to obtain 4V class voltage battery having high energy density, implementation of a lithium-ion secondary battery that uses a lithium composite oxide having layered structure or spinel structure as the cathode active material is advancing.
Currently, as the lithium composite oxide that is used as the cathode active material of a lithium-ion secondary battery, lithium cobalt composite oxide (LiCoO2) capable of being synthesized with relatively ease, lithium nickel composite oxide (LiNiO2) that uses nickel that is less expensive than cobalt, lithium nickel cobalt manganese composite oxide (LiNi1/3Co1/3Mn1/3O2), lithium manganese composite oxide (LiMn2O4) that uses manganese, and the like have been proposed.
In order to produce a battery having excellent performance such as a high cycling characteristic, low resistance and high output, it is necessary that the cathode active material be made up of particles having a uniform and suitable particle size. This is because, when cathode active material having a large particle size is used, it is not possible to sufficiently maintain surface area for reacting with the electrolyte, and there is a possibility that the reaction resistance of the cathode will increase and that it will not be possible to obtain a high-output battery; however, when a cathode active material having a very small particle size is used, the packing density of the cathode decreases, and there is a possibility that battery capacity per volume will decrease. On the other hand, when a cathode active material having a wide particle size distribution is used, the voltage that is applied to the particles of the cathode active material inside the cathode is not uniform due to the differences in particle size, and there is a possibility that problems will occur such as a selective deterioration of minute particles due to repeated discharging and charging, a decrease in battery capacity and an increase in reaction resistance of the cathode.
Moreover, in order to achieve a battery with higher output, shortening the distance that lithium ions move between the cathode and the anode is effective, so making the cathode plate thin is preferable. From this aspect, using a cathode active material having a small particle size within a range in which the voltage capacity per volume does not decrease is useful.
From the aspect of further increasing the output of the battery, not only is it necessary to use a cathode active material having a uniform and suitable particle size, but it is also necessary to use a cathode active material having a high specific surface area. For example, it is possible to increase the specific surface area by lowering the smoothness of the particle surface of the cathode active material, or by using particles having a porous structure, even when the particle size of the particles is kept the same. In that case, the reaction surface area of the particles and electrolyte becomes large, and it becomes possible to increase the reactivity of each, so it becomes possible to further improve the output of the battery.
In order to improve the performance of the lithium-ion secondary battery in this way, it is necessary to produce a lithium composite oxide as the cathode active material so as to have a uniform and suitable particle size, and so that the particles have a large specific surface area.
JP2008-147068 (A) discloses a lithium composite oxide of which in the particle size distribution curve, the particles have an average particle size D50, which means the particle size having a cumulative frequency of 50%, of 3 μm to 15 μm, and a particles size distribution having a minimum particle size of 0.5 μm or greater and a maximum particle size of 50 μm or less, in the relationships with the cumulative frequencies of 10%, D10, and 90%, D90, D10/D50 is 0.60 to 0.90, and D10/D90 is 0.30 to 0.70. This lithium composite oxide has a high packing characteristic, good discharging and charging capacity characteristic, and a high output characteristic, and deterioration is difficult even under conditions of a large discharging and charging load, and by using this lithium composite oxide as a cathode active material, it is possible to obtain a lithium-ion secondary battery having an excellent output characteristic and little deterioration of the cycling characteristic.
JP2004-253174 (A) discloses a lithium composite oxide that has a layered structure that comprises hollow particles having an outer shell on the outside and a hollow section inside this outer shell. Cathode active material comprising this kind of lithium composite oxide has an excellent cycling characteristic, output characteristic, heat stability and the like, and can be suitably used in a lithium-ion secondary battery.
However, even though the lithium composite oxide that is disclosed in JP2008-147068 (A) has an average particle size of 3 μm to 15 μm, the minimum particle size is 0.5 μm or greater and the maximum particle size is 50 μm or less, so there is a mixture of minute particles and rough particles, and from the values of D10/D50 and D10/D90 above, a narrow particle size distribution range is not possible. In other words, the lithium composite oxide disclosed in this literature cannot be said to have particles having a uniform particle size, so even when this lithium composite oxide is used as a cathode active material, it is difficult to improve the performance of the lithium-ion secondary battery.
Moreover, the lithium composite oxide that is disclosed in JP2004-253174 (A) has hollow particles, so the specific surface area is expected to be greater than solid particles, and it is thought to be possible to improve the reactivity of the particles and electrolyte due to the increased specific surface area. However, this literature says nothing about the particle size and particle size distribution of the lithium composite oxide. Therefore, it can be considered that this lithium composite oxide does not take into consideration the particle size and particle size distribution, so there is a possibility that selective deterioration of particles due to non uniform voltage being applied inside the electrodes caused by a non uniformity of particle sizes will occur, and that a decrease in battery capacity is not avoidable.